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SUMMARY The oz1 ive11 surveyinq ran he carried out by means of the multiple shot instrument or the sinqle shot instrument. The srrond type instrument is Maqnetic. It is possible to pump Criell? I kinks I) are present. Enumeration of the numerous advantaqes of the controlled direcfionnl drillinq.
Summary. A measurement-while-drilling (MWD) directional survey tool and a set of magnetic single-shot instruments used while drilling a particular, highly deviated well were removed from the field and put in a controlled laboratory environment. Tests were conducted to compare directional-survey azimuth readings obtained with the single-shots against azimuth readings taken with the MWD system. We found both measuring systems were generally accurate and repeatable; but we observed differences in azimuth readings that sometimes exceeded l degree under even ideal test conditions. The MWD tool was more repeatable and, on average, somewhat more accurate.
Oil company personnel and directional drillers are growing more accustomed to using mud-pulse telemetry MWD tools for directional surveying in lieu of magnetic single-shots (which had dominated the directional measurement market since 1934). Frequently, a directional driller will run a single-shot on top of the MWD tool to obtain a crosscheck. But MWD systems and single-shot devices do not always produce identical azimuth readings downhole, and this leads to questions about azimuth differences. (Usually, discrepancies in inclination-angle readings are quite small or nonexistent.)
An opportunity to examine differences between MWD and single-shot survey readings arose when Aminoil USA questioned some azimuth-angle discrepancies observed during drilling. Both the oil company and Teleco Oilfield Services Inc., the MWD company, decided that ideally the single-shot instruments and the MWD system in question should be compared under the best possible conditions to eliminate extraneous causes of discrepancies. One such factor is magnetic interference caused by the drillstring. When a single-shot is run on top of an MWD system, the distance from each of the compasses (in the case of the MWD system, its "compass" is really a magnetometer) to the nearest magnetic metal in the drillstring is different. In some circumstances, the unequal distances can cause differences in azimuth readings between the two systems. Other factors that exist in the field include (1) alignment of the survey instrument within its drill collar, (2) alignment of the drill collar in the borehole, (3) whether the specific nonmagnetic drill collars in use have any magnetic hot spots that are located where they could affect azimuth readings, and (4) how precisely one can read the single-shot film. We did not want to test laboratory-specimen equipment but rather to test the specific hardware used in drilling the well that triggered the study. Therefore the MWD system and the single-shot instruments used to drill the well were set aside specifically for this study. Neither the MWD system nor the single-shot devices received any special tune-up or other special preparation before the tests.
We conducted this test in a laboratory environment created specifically for testing magnetic survey instrumentation. We wanted to be sure any observed differences were attributable strictly to the survey tools rather than our test procedures or equipment. procedures or equipment. Nonmagnetic Environment. Our tests were performed in Teleco's sensor calibration building in Meriden, CT, a structure designed and built to be free of magnetic anomalies. When this building was constructed, magnetic field surveys were made before the site was selected, several times during construction, and again after construction to ensure that the building remained free of any magnetic field problems. The building has wooden beams instead of steel beams, concrete without steel reinforcement, aluminum nails instead of iron, plastic conduit, etc. Similar buildings are used for directional sensor calibration at our field maintenance facilities.
Aligmment of Test Fixture. The test fixture must be accurately aligned relative to magnetic north. A Model DIM-100TM inclination and declination magnetometer is used for this purpose. The DIM-100 is a precision instrument of the same type used by the U.S. Dept. of the Interior Geomagnetic Survey Team.
ABSTRACT This paper discusses some of the outstanding developments made in the drilling and completion technique during the last decade. Following a brief outline of the history of the development of various well-surveying instruments during the past 60 years, the most necessary requirements for instruments and surveying methods are stated. The various types of instruments, their operation, the correction of magnetic-compass records, the preparation of survey maps, and the frequency of taking records are described. The development of directed controlled drilling, its objectives and requirements, are explained. A brief description of the various tools used in this art of drilling and their method of use is given. Two principal methods of obtaining side-wall core samples in connection with electrical logging are described. A new method of determining the direction and the angle of the dip of the formation by orienting core samples off the core trays is depicted. INTRODUCTION In recent years there have been placed at the disposal of the oil-production industry a large number of instruments and tools, the use of which made the operators abandon the rule-of-thumb practices of long standing and substitute for them improvements in drilling and completion technique, with a far greater recovery of the oil underground than formerly was considered possible. Some of the outstanding developments during the last decade would include the following: Well-surveying instruments. Controlled directed drilling. Side-wall sampling. Determination of the direction of dip of core samples by using the residual magnetism of minerals in the core sample. A brief resume of these developments, the factors which should be considered when they are employed, and their coordination with the improvements of the drilling technique are presented herewith. Underground Well Surveying The art of well surveying is not so recent as might be supposed. More than 60 years ago mining engineers recognized the problems encountered when a well or test hole is crooked. The first known instrument for detecting the deflection of a bore hole was introduced in 1873 by G. Nolten, Dortmund. This comprised a glass bottle in which was placed an acid of sufficient strength to etch the interior of the glass when allowed to remain stationary for a predetermined length of time. Inasmuch as the axis of the bottle was parallel to the axis of the hole and the surface of the acid was horizontal, the angle of inclination from the vertical could be calculated. In conjunction with this instrument a compass was used, the needle of which could be arrested after a predetermined lapse of time. This instrument permitted only single readings. In 1884 E. F. MacGeorge brought out and tested at Sandhurst in Victoria, Australia, an instrument which marked a significant advance upon all preceding methods. This instrument, called a clinostat, consisted of a glass phial, in the bottom of which was a compass needle and in the top a small glass plummet. The phial was filled with hot transparent gelatin and lowered into the well. The density of the plummet and of the float supporting the needle was designed in such a way that they both would float freely in the hot gelatin.